hey maurizio…the .mlt file is a project file created by Shotcut as a placeholder of sorts…it is not the actual video you are trying to create. the .mlt is associated with all of the image files that are the actual source of the video you are creating. typically what i will do is have the .mlt saved in the same location as the source files i’m using to create a video…on a side-note, (to help you out before you become frustrated), when animating with Shotcut i’ve found it best to select one of the images you are working with by double clicking it, then opening the properties tab, (that is if you have added the properties tab to your playlist window), and reducing the time for the image from 4 seconds, (the default setting), to .01 seconds per image and then saving that as the default setting. you can change it back to whatever length you like for other projects…but when animating i’ve had more than satisfactory results from this setting…just sharing the wealth, hope this helps. there really isn’t an actual help file with the program, but the community here is more than willing to help. a good place to read up on the program is found at https://www.shotcut.org/howtos/
The easiest way to insert all your single frames into the timeline is to drag and drop the all clips/stills to the playlist. Then use the Add All to Timeline function to add them to the timeline.